When a structure vibrates, for example, machinery or a building having an engine or a motor vibrates, a vibration damping apparatus damps vibration by adding anti-phase vibration of the vibration. For example, the vibration damping apparatus disclosed in JP 2005-299832A adds anti-phase vibration of the vibration generated in an engine serving as a vibration source by use of an electromagnetic actuator. Further, the vibration damping apparatus is designed to carry out damping depending on an operation state of the engine. That is, the vibration damping apparatus drives the electromagnetic actuator based on signals, and the signals are derived from different methods which are selectively changed between the damping for low frequency vibration and the damping for high frequency vibration. More specifically, the low frequency vibration occurs when the vehicle provided with the engine is in an idle state and the high frequency vibration occurs when the vehicle is in a running state.
The vibration source such as an engine is mounted on a fixed end. For example, the vibration source is fixed to a vehicle body. Thus, a vibration system which is composed of the fixed end and the vibration source has a resonance frequency. When the vibration source vibrates at the resonance frequency or at similar frequencies, the vibration is amplified and the amplitude becomes larger. The resonance frequency changes depending on how the vibration source is connected to the fixed end (for example, the connection via springs) or mass of the vibration source. Thus, as disclosed in JP 2005-299832A, it is not possible to deal with the resonating vibration system by merely changing the control between the idle state and the running state.
If the electromagnetic actuator which generates an anti-phase vibration generating force is sized up, then it is possible to obtain output power enough to damp the amplitude of the vibration system in a resonant state. However, in that case, the cost of the apparatus increases and it would be difficult to secure a space to dispose the apparatus due to size increase of the vibration damping apparatus provided with the electromagnetic actuator. Further, if the size of the vibration damping apparatus is increased, it is not possible to adequately utilize the ability of the vibration damping apparatus when the vibration source vibrates at the frequency other than the resonance frequency and thus the amplitude of the vibration system is small.
On the other hand, the size of the vibration damping apparatus remains small, when the vibration source vibrates at the frequency other than the resonate frequency, it is possible to carry out the effective damping taking full advantage of the ability of the apparatus. However, when the vibrating source vibrates at the resonance frequency, even though the apparatus attempts to carry out the damping, it is not possible to carry out the damping effectively due to large amplitude caused by the resonance leading to loss of electric energy required for attempting the damping.
The present invention has been made in view of the above circumstances, and provides a vibration damping apparatus which is able to carry out the effective damping without the loss of the energy therefore.